codegen.c

#include <stdio.h>
#include <string.h>
#include <sys/mman.h>

typedef int (*ifunc)();

unsigned char icode[] = {   0x90, 
                            0xB8, 
                            0x00, 0x00, 0x00, 0x00,
                            0xC3
                        };

int main()
{
    *(int *) (icode + 2) = 42;
    void *xcode = mmap(0, sizeof(icode),
                        PROT_READ | PROT_WRITE | PROT_EXEC,
                        MAP_PRIVATE | MAP_ANON, 0, 0);
    memcpy(xcode, icode, sizeof(icode));
    ifunc fcode = (ifunc) xcode;
    printf("%d\n", fcode());
    return 0;
}